Electronics and Embedded Programming V2 - Developers

[QUOTE=DrLuke;33549257][img]http://i54.tinypic.com/2ex26gh.png[/img] 4 of these lasers für 3 dollars on dealextreme. I didn't expect much, but they make a very nice dot with almost no noise around it.[/QUOTE] What is the part number for these?

[QUOTE=ddrl46;33601521]You can get everything for free on their website: [url]http://www.nerdkits.com/videos/[/url] [url]http://www.nerdkits.com/projects/[/url][/QUOTE] That's just the projects, there also is a site long PDF document teaching all about AVRs and basic electronics.

Finished reading most of "Build your own Z80 Computer." Do I need everything in the book? I'm thinking that for a start, the only input/output I should have is an UART and that I can just scrap the keyboard and LED display, as those seem to just over-complicate things.

[QUOTE=amazer97;33605102]Finished reading most of "Build your own Z80 Computer." Do I need everything in the book? I'm thinking that for a start, the only input/output I should have is an UART and that I can just scrap the keyboard and LED display, as those seem to just over-complicate things.[/QUOTE] If you're making it to just function as a controller of some sort then most likely not, that guide is more on how to make a bare-bones general-purpose computer.

So I'm looking to build a [url=http://www.pittnerovi.com/jiri/hobby/electronics/welder/index.html]capacitive discharge welder[/url] - I've come across a some details I want to double check before possibly conjuring up gratuitous amounts of magic smoke. Please note that the only reason I'm posting this is because I want constructive criticism. The only experience I've had with circuit building before is in logic design labs - that said, I do know what I'm talking about now is at least an order of magnitude more dangerous if I'm uninformed. I've gleaned information from two schematics: [url=http://www.pittnerovi.com/jiri/hobby/electronics/welder/schematics1.pdf]One (pdf)[/url] [url=http://www.pittnerovi.com/jiri/hobby/electronics/welder/franz-welder.sch]Two (sch)[/url] Questions: Q1. An 820R 1206 film resistor is 820 ohms, right? "R" sure is a funny way to spell Ω. Following from this, a 1R resistor would be 1 ohm. Would it be considered safe to discharge a 1.0 farad 20 volt capacitor by using said 1 ohm 5 watt resistor? From quick calculation, the first tenth of a second would see a voltage drop of about 1.8 volts and a current of about 1.8 amps; (ab)using calculus, the maximum power at any point would be about 4 watts and thus safe, correct? As an aside, perhaps I could use cheaper MOSFETs for charging and slow discharging... Q2. I am assuming that 1.0μF / 100μF tantalum capacitors for bypasses are not worth the extra cost over aluminum electrolytic for this project - am I wrong? Q3. I'm going to use a 1.0μF aluminum electrolytic capacitor and a large resistor in series in place of a 1.0F capacitor to test the circuit to avoid magic dust. I'll probably leave out 5 of the MOSFETs in such a test case - those bastards are expensive. Any problems here? Q4a. Concerning the rapid discharge aspect: What gauge wire should I use? 8 AWG? 4 AWG? Also, I'm going to be using 6x IRFP2907ZPBF MOSFETs in parallel to rapidly discharge a 1.0 farad 20 V capacitor for the welding process. What's the best way to connect the pins to the large diameter wire - solder to a smaller intermediate gauge wire first? Any tips, then, on soldering different gauges of wire? Q4b. For cooling the MOSFETs, is eight of [url=http://www.mouser.com/ProductDetail/Wakefield/657-20ABP/?qs=sGAEpiMZZMttgyDkZ5Wiuh6vt2S17U8%2fWe7E3KuKB%252bA%3d]these heatsinks[/url] too little, just enough, or can I put two MOSFETs to a single heatsink? Q5. Darlington transistors double voltage, correct? It seems like the schematic I'm using exploits this to route a 5V logic signal to a transistor that has 15V on the emitter (the transistor being used has a breakdown voltage for emitter - base = -5 V, guessing this has something to do with it). I want to fully understand what I'm building :v: Q6. Power supply - I need 20V to charge the capacitor, 15V to drive the MOSFETs, and 5V for logic. I assume I'm best off building my own solution rather than sourcing three separate power supplies. From a little research, I think it would be best for me to not build a SMPS. Tell me if I'm on the right track: [IMG]http://i.imgur.com/Pqmq8.png[/IMG] T1. Use a transformer 120 VAC -> 24 VAC (slow blow fuse in series with primary coil) T2. Use [url=http://www.mouser.com/ProductDetail/Central-Semiconductor/CBR35-010P/?qs=8DZ%2fSIieAlOX9gLc%2f1PPlw%3d%3d]CBR35-010P[/url] 70V peak 35A bridge rectifier (24V * sqrt(2) * 2 = 68V peak inverse voltage) and 10 000μF capacitor for 24 VDC T3. Use linear regulator LM7805 on unregulated 24VDC with 1.0μF capacitor for 5 VDC T4. Use linear regulator [url=http://www.mouser.com/ProductDetail/STMicroelectronics/L7815ACV/?qs=sGAEpiMZZMug9GoBKXZ75xbXFDWiKqpm6LxYrwrCEjs%3d]L7815ACV[/url] on 24VDC with 1.0μF capacitor for 15 VDC T5. Charge 1.0F capacitor with (unregulated) 24 VDC until ATMEGA16 detects the desired voltage has been reached Should I have a separate transformer and/or bridge rectifier for each voltage/charging voltage instead? Also, my project should be connected to the earth ground rather than floating, right? Concern: Would either linear regulator need to be on a heatsink? I'm guessing the 15V doesn't need one since it is only driving the gate of 6 MOSFETs through a transistor (i.e. low current). Also, should I use a 20V linear regulator for charging the capacitor e.g. [url=http://www.rohm.com/products/databook/general/pdf/ba178xxt-e.pdf]BA17820[/url]? I'm not sure if the output voltage from the bridge rectifier is high enough.

[QUOTE=Night-Eagle;33609834]gibberish[/QUOTE] Wouldn't running the LM7805 off the LM7815 be a little more efficient?, i am not entirely sure as it would draw more current through the LM7815 but would drop less on the LM7805.... Someone, do calculations!

[QUOTE=amazer97;33605102]Finished reading most of "Build your own Z80 Computer." Do I need everything in the book? I'm thinking that for a start, the only input/output I should have is an UART and that I can just scrap the keyboard and LED display, as those seem to just over-complicate things.[/QUOTE] The Z80 itself doesn't have a built-in UART, or any peripherals at all - you need to have some sort of peripherals that will let you memmap this hardware onto the main address space. I know Zilog introduced some official peripherals to go with the Z80, including a some timers, DMA hardware and serial I/O, but you can probably hack these together using a bunch of 74xxx or 4xxx chips.

[QUOTE=SubbyV-2;33610040]Wouldn't running the LM7805 off the LM7815 be a little more efficient?, i am not entirely sure as it would draw more current through the LM7815 but would drop less on the LM7805.... Someone, do calculations![/QUOTE] It would be more or less the same in total. [editline]7th December 2011[/editline] But maybe it's better to get a bigger cap before, and one after each of the voltage regulators.

[QUOTE=SubbyV-2;33610040]Wouldn't running the LM7805 off the LM7815 be a little more efficient?, i am not entirely sure as it would draw more current through the LM7815 but would drop less on the LM7805.... Someone, do calculations![/QUOTE] It would be exactly the same, :(.

For those who like Lua, here's a [URL="http://www.eluaproject.net/"]Lua for µC's[/URL] ([URL="http://wiki.eluaproject.net/Boards"]supported boards[/URL])

Useless unless you have a lot of cpu power and RAM available.

[QUOTE=q3k;33610097]The Z80 itself doesn't have a built-in UART, or any peripherals at all - you need to have some sort of peripherals that will let you memmap this hardware onto the main address space. I know Zilog introduced some official peripherals to go with the Z80, including a some timers, DMA hardware and serial I/O, but you can probably hack these together using a bunch of 74xxx or 4xxx chips.[/QUOTE] Yeah, I understand that but what I'm asking is do I really need any more other than an UART chip? I could hook it up through an IO decoder and directly to the data bus and use it like that.

[QUOTE=Night-Eagle;33609834]Long post[/QUOTE] I'll try answer your questions: Q1 - Yes 820R is 820 Ohm and 1R is 1 Ohm, if you see something like 2R2 then that means 2.2 Ohm. (R being equivalent to the decimal point) As for discharging a 1F capacitor charged to 20V through a 1 Ohm 5W resistor this should be no problem as long as you don't repeat it too often, the peak instantaneous power dissipation is 400W which will fall to roughly 54.76W after one second or 8W after two seconds. Q2 - Standard aluminium electrolytic will work fine Q3 - No problem, but you don't need a series resistor Q4a - Use thick solid copper wire and or stranded cable, the thicker it is and the more strands the lower the resistance, as for the soldering you want to get the thick wire as close as possible to minimize added resistance, if you're using strip board use multiple traces. Q4b - The heatsinks should be fine since they have a chance to cool down between discharges, but I'm no expert when it comes to thermal calculations. Q5 - Darlington transistors have double the base-emitter voltage drop and a much higher current gain which reduces the required base current, transistors are usually never run in the reverse breakdown region so your probably reading the schematic incorrectly (I've not looked at it yet). Q6 - I can't quite see why you would need a separate 15V to drive MOSFETs, I took a quick look at the schematic and honestly it looks pretty poorly designed, yes it would most likely work but eh it's up to you. As for your power supply you designed, the 24V output will around 32V so you'll want a regulator regardless, leaving it up to the microcontroller to detect the capacitor voltage is bad design, the regulator is overkill (35A what?!) and you don't really need 10,000uF (2,200uF will do it) or C2. Oh and "(24V * sqrt(2) * 2 = 68V peak inverse voltage)" is incorrect, you don't multiply it by 2 that would give you the peak-to-peak voltage.

Cool, somebody on a forum is selling a MOSFET that can switch a HUGE current: [url]http://datasheetz.com/data/Discrete%20Semiconductor%20Products/MOSFETs%20-%20Arrays/VWM200-01P-datasheetz.html[/url]

[img]http://i.imgur.com/McOXU.png[/img] Electret mic amplifier as requested by Chipset, has a voltage gain of approximately 100 (40dB). I so love transistors.

[QUOTE=Chryseus;33612455]Q3 - No problem, but you don't need a series resistor[/QUOTE] I'll want to increase the RC time constant artificially so I can verify the charging circuit works correctly with just a multimeter :) [QUOTE=Chryseus;33612455]Q6 - I can't quite see why you would need a separate 15V to drive MOSFETs, I took a quick look at the schematic and honestly it looks pretty poorly designed, yes it would most likely work but eh it's up to you.[/QUOTE] If I've read the [url=http://www.irf.com/product-info/datasheets/data/irfp2907z.pdf]datasheet[/url] correctly and understand these MOSFETs, the resistance between the drain and source decreases as the gate voltage increases. Since we're pulsing the MOSFET, the extra gate voltage is needed for the rapid discharge - but figure 3 shows that a 60μs pulse of 10V will get 1000 A between drain and source. Perhaps the machine [url=http://www.pittnerovi.com/jiri/hobby/electronics/welder/index.html]this guy[/url] designed needs current in excess of 6000 A? I'm looking at [url=http://www.robot-welding.com/Welding_parameters.htm]this page[/url] for some insight, but the material listed there is very different from what I'm working with (0.12mm to 0.075mm). Let's see about interpolating that information by sight: [img]http://eagle.undo.it:8083/img/spotweld.png[/img] (For steel sheets, spot welding two of the same thickness sheets together) Looks like 6000 A is in the ballpark. Perhaps I should use 10V to drive the MOSFETs? In that case, the MOSFETs could just be driven directly from the Darlington Transistor - thus eliminating the need for a 10V linear regulator completely, yes? [QUOTE=Chryseus;33612455]As for your power supply you designed, the 24V output will around 32V so you'll want a regulator regardless, leaving it up to the microcontroller to detect the capacitor voltage is bad design, the regulator is overkill (35A what?!) and you don't really need 10,000uF (2,200uF will do it) or C2.[/QUOTE] Oh, wow. The transformer I was looking at was rated at 24 VA. Completely missed that. New question: The schematics I've looked at use really bulky transformers to charge the capacitor (500 VA). To save on cost with a smaller transformer, how can I limit the charging circuit's current dynamically to minimize charge time? If I regulate the charging voltage to 20 V and statically limit the current to 1 A with a 20 ohm resistor, charging to, say, 16 V would take way too long. Shouldn't be a problem for battery tab welding (6V), but I'll need this for "other stuff" too. [QUOTE=Chryseus;33612455]Oh and "(24V * sqrt(2) * 2 = 68V peak inverse voltage)" is incorrect, you don't multiply it by 2 that would give you the peak-to-peak voltage.[/QUOTE] Whoops - I was making that calc for a lone diode rather than a bridge rectifier. I'll use the 1:30 AM calculation excuse here :P Also, for some reason I believed the capacitor would hold the charge at V[sub]rms[/sub]. That was dumb - I was even [url=http://www.falstad.com/circuit/#%24+1+5.0E-6+0.9487735836358526+50+5.0+48%0Av+144+368+144+80+0+1+60.0+33.9411+0.0+0.0+0.5%0Aw+144+80+272+80+0%0Aw+272+80+272+144+0%0Ad+272+144+336+208+1+0.805904783%0Ad+272+272+336+208+1+0.805904783%0Ad+208+208+272+144+1+0.805904783%0Ad+208+208+272+272+1+0.805904783%0Aw+272+272+272+368+0%0Aw+272+368+144+368+0%0Aw+208+208+208+304+0%0Aw+336+208+384+208+0%0Aw+208+304+384+304+0%0Ac+384+208+384+304+2+1.02E-4+30.384667271174408%0Aw+384+208+464+208+0%0Aw+384+304+464+304+0%0Ar+464+208+464+304+0+430.0%0Ax+499+264+529+268+0+16+load%0Ao+0+32+0+34+40.0+0.4+0+-1%0Ao+15+32+0+35+40.0+0.1+1+-1%0A]staring at a graphed simulation[/url] of a filtered full-wave bridge rectifier's output :v: Is there a simple way to get the average V[sub]DC[/sub] output of a filtered bridge rectifier I'm missing? Or did you just take the V[sub]peak[/sub] and do a rough interpolation in your head / memorize that?

I've taken a good look over that schematic and I can honestly tell you it's one of the worst circuit designs I have ever seen, and I've seen some pretty bad stuff, I would advise you not to build that but to find a decent circuit instead, whoever designed that needs to go back to studying electronics. So I shall point out the problems I have found so far: Use of three transformers which is totally not needed, in addition the 500VA transformer is a bad idea, not only is it expensive and heavy but a 100VA 15V transformer with a 4.7ohm charge resistor will work just as well, you can also power the drive circuitry from the charging supply. The discharge circuitry is not really needed, you can simply weld nothing to discharge the capacitor. Unnecessary darlington transistor array IC, since Vdrive is driving a MOSFET there is almost no current due to the high gate impedance, in other words you can use a single general purpose BJT or MOSFET. Excessive power supply decoupling capacitors where one would do the job. Also there is absolutely no need for a microcontroller in such a simple circuit, the entire thing can be done with a few comparators and transistors. Might I ask where you are getting a 1F 20V from, they're damn expensive...

I think the discharge circuit isn't a bad idea at all, it prevents accidents.

[QUOTE=DrLuke;33628227]I think the discharge circuit isn't a bad idea at all, it prevents accidents.[/QUOTE] Accidents ? Your hardly going to hurt yourself with 20V

[QUOTE=Chryseus;33628323]Accidents ? Your hardly going to hurt yourself with 20V[/QUOTE] When you're sweating, 20V can be dangerous.

[QUOTE=Chryseus;33628323]Accidents ? Your hardly going to hurt yourself with 20V[/QUOTE] A spark from an accidental discharge could burn you or set something on fire.

[QUOTE=yngndrw;33629930]A spark from an accidental discharge could burn you or set something on fire.[/quote] Usually when using a spot welder you don't put your hand or anything flammable near it, and there is still the issue of how to trigger the discharge, either do it manually via a switch (which solves nothing if the person forgets) or on a timer which can be inconvenient. A spot welder is a tool not a toy, that means proper safety precautions should be taken when using it. The best solution I think would simply be a charged indicator LED, and if someone ignores it and gets hurt, well that's their own fault.

[QUOTE=Chryseus;33631583]Usually when using a spot welder you don't put your hand or anything flammable near it, and there is still the issue of how to trigger the discharge, either do it manually via a switch (which solves nothing if the person forgets) or on a timer which can be inconvenient. A spot welder is a tool not a toy, that means proper safety precautions should be taken when using it. The best solution I think would simply be a charged indicator LED, and if someone ignores it and gets hurt, well that's their own fault.[/QUOTE] Or just get a discharge circuit.

A parallel resistor ? Well if you used a 2.2k 1/4W resistor in parallel it would still take 3 hours to discharge, unless you use a small power resistor but that's extra power being wasted and additional cost for the resistor. I don't see the issue of it being left charged.

You could of course use a P-channel MOSFET to discharge it when the main power is turned off or something.

What do you guys think of the [URL="http://www.raspberrypi.org/"]Raspberry Pi[/URL]? It's a $25-35 Linux PC the size of a credit card, GPIO headers, 128 MB of RAM ($25 model) or 256 MB of RAM with Ethernet ($35). There's two flaws that I could think of right now, which is the lack of a real time clock and that you would need a converter (Around $30 on eBay) to use it with a VGA monitor.

The video output is really the only reason why I am still thinking about getting one, otherwise it's a brilliant platform. But my guess is that it will be out of stock for a loooong time with a long backorder.

[QUOTE=Chryseus;33627803]I've taken a good look over that schematic and I can honestly tell you it's one of the worst circuit designs I have ever seen, and I've seen some pretty bad stuff, I would advise you not to build that but to find a decent circuit instead, whoever designed that needs to go back to studying electronics.[/QUOTE] Perhaps he could go back to studying quantum chemistry :) When I read his (Pittner's) little homepage, I realized I should be pretty skeptical of his design - I'm gleaning information off of these schematics to build my own schematic. I wouldn't build anything I'm not confident about; hence why I'm asking competent people. To be fair, though, it seems that Pittner referenced that PDF from a colleague in the U.K. - it isn't clear if they collaborated. That said, the idea behind the discharge circuit is to quickly allow welding at lower voltages after welding at a high voltage (the MCU will discharge the capacitor until it reaches the target voltage). I agree - not necessary, but when you already have a microcontroller in, why the hell not? The added cost is almost trivial. [QUOTE]Unnecessary darlington transistor array IC, since Vdrive is driving a MOSFET there is almost no current due to the high gate impedance, in other words you can use a single general purpose BJT or MOSFET.[/QUOTE] I don't understand how a BJT would work here. The way he has it set up, 5V, or supposedly 10V from the ULN2003A, is on the base of a PNP transistor and 15V is on the emitter. Now, wouldn't the base-emitter junction always be forward-biased? Is it even possible to switch a high voltage with a lower voltage via a single BJT? This is driving me nuts; I can't find a single example of this (other than the schematic for this project). I probably just don't understand transistors. I think I might just need a MOSFET to drive my MOSFET array. I either have to run the 6x MOSFET array at 10V, or buy 60 MOSFETs ([url=http://www.irf.com/product-info/datasheets/data/irfp2907z.pdf]see figure 3[/url]). [QUOTE=Chryseus;33627803]Also there is absolutely no need for a microcontroller in such a simple circuit, the entire thing can be done with a few comparators and transistors.[/QUOTE] Approximately how many comparators and transistors would I need to implement the welding process: 1. Charge to voltage set by pot 1 1a. Discharge/recharge to voltage set by pot 1 if user has changed pot 1 2. On signal, open gate on 6x MOSFET array 3. Wait 0.5 ms (adjustable by pot 2) 4. Re-open MOSFET gate 5. Wait 2 ms (adjustable by pot 3) 6. Close gate 7. Wait 4 ms (adjustable by pot 4) 8. Re-open gate 9. Repeat I'd also need some way of displaying and verifying those values, too (for safety reasons - blowing up a Li-Ion battery doesn't sound pleasant)... Also, I'm definitely a programmer before an electrical engineer, and I'd like to have some experience with MCUs. [QUOTE=Chryseus;33627803]Might I ask where you are getting a 1F 20V from, they're damn expensive...[/QUOTE] Apparently, [i]some[/i] automotive capacitors actually meet their advertised specs. I didn't believe it either, but I've found several independent sources (forum posts, YouTube videos) confirming this - all using these products in capacitive discharge welders. Also worth noting is that [i]most[/i] automotive capacitors come nowhere near meeting their advertised specs. Which is more surprising is a matter of opinion, but I intend to further "test" these claims ;) Oh, and thanks Chryseus. You've motivated me to self-study and lose more sleep over the last three days than I have for any final exam I've ever taken - seriously, there's just so much shit I know absolutely nothing about.

Finally got the last parts for the oscilloscope and soldered them in, ready to plug it in after school!

I had a quick go at designing a spot welder including a power supply using minimal components: [img]http://i.imgur.com/iJ1q3.jpg[/img] I see no reason why this should not work, although I've not tested it yet. The supply is a simple linear regulated supply, not the most stable (will droop a little under load) but good enough. When it is first switched on the Voltage on the op-amp output will be zero assuming the reference voltage pot in not turned right down. The transistor on the far right is off which works as an inverter turning on the charging MOSFET, once the capacitor reaches the reference voltage set by the pot the op-amp output goes to 20V turning off the charging MOSFET. When the discharge switch is closed the cap discharges and the op-amp once again goes to 0V and charging starts again. [b]Edit[/b] Found and fixed a possible problem: [img]http://i.imgur.com/RuJOc.png[/img] Added MOSFET to voltage reference which without could cause it not to recharge or discharge fully. Adjusted drain resistor of inverter to increase gate voltage to 18.1V from 10V Oh and the new MOSFET does not need to be a power MOSFET despite the text.